Hrm. What version of Java are you running? Note that Java 8 is no longer sufficient in all of its flavors -- see this thread that I posted this morning.
This random stackoverflow post suggests it's likely a version issue. If you can try to run it from the command line (e.g. PowerShell -> java -jar KoLmafia-25714.jar) you might be able to get more information.
In particular, if you get something like UnsupportedClassVersionError as noted in a screenshot on that post, that means that you're trying to run with an older version of Java.